Pathology

Tumours

 

The term "brain tumour" usually includes all neoplastic growths originating from the skull, the meninges, cranial nerves, pituitary, brain parenchyma, embryological remnants, metastases and lymphomas. Many classifications have been proposed mainly based on histological criteria, taking into consideration the cell of origin and the degree of malignancy; periodical revision of these criteria by neuropathologists, adds complexity to the histological classification.
For the purposes of this chapter we will adopt a main basic topographical subdivision within which the histological subtypes will be considered:
supratentorial tumours
    extraaxial
    intraaxial
infratentorial tumours
    extraaxial
    intraaxial
Pituitary and parasellar tumours are treated in a specific chapter.

Different histological subtypes occur more frequently in the paediatric age (infratentorial tumours, medulloblastomas, spongioblastomas, ependymomas), while others are more frequent in adults and in the supratentorial compartment (meningiomas, gliomas).

The main aim of the neuroradiological diagnosis is first to identify and recognise the tumour, then to locate it precisely, de fine its gross macroscopic structure (solid, cystic, necrotic, calcified, etc) and recognize its relationships with the surrounding structures (mass effect, hydrocephalus). To achieve this task the three main diagnostic modalities, CT, MRI and angiography may be necessary.

MRI is the most informative modality due to its multiplanar capabilities that provide superb topographic details as well as its multiparametric specificity (T1, T2, Proton Density Weighted Images) that provides maximum contrast resolution; CT, however, better demonstrates calcification, providing relevant information on age and gross characteristics of the tumour.
Angiography is nowadays very rarely necessary for diagnostic purposes; it is performed only when knowledge of the type and degree of vascularization is relevant for planning surgery or when a preoperative embolisation may be envisaged, for example meningiomas.

Supratentorial, extraaxial tumours

They are mainly represented by meningiomas, craniopharyngiomas, epidermoids, chordomas, neurinomas.
Only meningiomas will be discussed in this chapter, since the other histological types occur mainly at the level of the skull base and in the sellar region where they will be dealt with.

Meningiomas
Meningiomas are the most common, non glial, primitive intracranial tumours; their prevalence among operated tumours is around 13-19%. They may occur at any age but have a peak incidence around 45 years of age; 60% occur in females. One% are multiple, usually in neurofibromatosis.

The most common locations are: falx and parasagittal (25%), convexity (20%), sphenoid (20%) olfactory groove (10%) suprasellar (10%), posterior fossa (10%), middle fossa (3%) and intraventricular (2%).

Neuropathology
The cells of origin of meningiomas are arachnoidal meningothelial; dural fibroblasts and pial cells may also participate in the formation of the meningioma. Macroscopically, they are usually well circumscribed, homogenous, with a capsule; sometimes they are necrotic, contain cystic components or areas of calcification.

They are usually subdivided from a histological point of view into five groups; syncitial, transitional, fibrotic, angioblastic and malignant.

Clinical presentation
Meningiomas may present with seizures, progressive focal neurological signs in relation to the location of the tumour and signs of raised intracranial pressure.

Neurological signs may appear very late, when tumours have reached a very large size.

Neuroradiological diagnosis
Plain films of the skull may be diagnostic when they show focal hyperostotic changes of the skull or calcification, associated or not with signs of raised intracranial pressure (erosion of the floor of the sella turcica).

CT is almost invariably able to demonstrate the presence of the meningioma if the examination is performed using intravenous contrast enhancement.

Since meningiomas are usually isodense to the cerebral cortex, small meningiomas located in critical topographical positions such as the floor of the middle fossa, may be missed if only axial slices without contrast injection are acquired.

About 20 % of meningiomas contain areas of calcification and may then range from slightly to markedly hyperdense (Fig. 24, 25). Rarely meningiomas contain necrotic, cystic or fatty components. Almost invariably meningiomas enhance intensely and homogeneously following intravenous injection of iodinated contrast. Surrounding oedema may be absent or very marked, without definite relationship to tumour size.
On MRI (Fig. 24, 25) meningiomas tend to be isointense to cortex and hypointense to white matter in T1-weighted images; in T2-weighted images meningiomas are again isointense to slightly or markedly hyperintense.

Enhancement with Gadolinium (Gd) is usually very marked and homogeneous. MRI has the advantage over CT of being able to provide images in different planes; coronal images are very useful in demonstrating critical areas such as the middle fossa or the upper convexity; MRI shows much better than CT the relationship of the tumour with vascular

a   b c   d Figure 24. Large, calcified falx meningoma a, b) CT without and with contrast injection. Markedly hyperdense (calcific) anterior midline space occupying lesion, with enhancement. c) MRl, T2-weighted image, axial. Inhomogenous mass with very low signal (calcifications) surrounded by some hyperintensity (oedema) of the left frontal lobe. d) MRl, T1-weighted image, coronal slice following Gd injection. Enhancement of the parenchymatous part of the meningiomas.

 

a   b c   d Figure 25. Right pterional meningioma a, b) MRI, T1-weighted image, in the coronal plane, without and with i. v. injection of Gd. Large, slightly hypo-isointense mass, displacing the frontal lobe, with marked inhomogeneous enhancement. c, d) Right external and internal carotid angiography, arterial phase. Hypertrophy of the middle meningeal arteries that provide blood supply to the tumour. Some supply is also provided by branches of the middle cerebral arteries.

structures, the carotid siphon, venous sinuses, their compression or obstruction and all topographical relationships useful for surgical planning. MRI is the modality of choice but CT is definitely diagnostic in the majority of cases.

Neither with CT nor MRI, despite certain characteristic features, is it possible to distinguish the different histological subtypes of meningiomas.

Once the extraaxial location is recognized, the differential diagnosis includes metastases and neurinomas.

Angiography (Fig. 25) is performed preoperatively not for diagnostic purposes but to evaluate the type and degree of vascularization, and with a view to possible preoperative embolisation.

Intraventricular meningiomas are a rare entity; they occur mainly within the lateral ventricles, at the level of the glomus of the choroid plexus in the trigone. They may mimic an intraaxial thalamic tumour and must be suspected any time a tumour growth is within the trigone, with sequestration and dilatation of the temporal horn, limited or no oedema of the surrounding parenchyma.

Supratentorial intraaxial tumours

Gliomas
Primary cerebral gliomas represent about 50% of all intracranial tumours; they include
1. astroglial tumours; the precursor cell type is the astrocyte
2. oligodendroglial tumours, originating from the oligodendroglia
3. ependymomas, originating from the ependymal cells
4. glioblastoma multiform, highly malignant glial tumour without a clearly defined cell of origin

Not infrequently mixed cell types are encountered. From a clinical point of view the most common presentation is that of progressive focal neurological deficits related to the localisation of the tumour; motor, sensory, visual fields, language, memory, behaviour etc. Signs of raised intracranial pressure or seizures are not infrequently the presenting sign.

Astrocytoma
In adults they represent 25-35 % of all supratentorial gliomas. They may arise anywhere in the cerebral hemispheres; histologically low grade gliomas are subdivided into three different types: fibrillary, protoplasmatic and gemistocytic.

At CT (Fig. 26) they appear as homogenous areas of hypodensity with relatively well defined margins, they rarely present with perifocal oedema. Contrast enhancement is rarely found.

a    b  c    d  e    f Figure 26. Right fronto-temporo-insular astrocytoma. a, b) CT without and with contrast injection. The tumour is hypodense and shows no enhancement. The two arrows point to the middle cerebral artery branches within the sylvian fissure. c) MRI, T1-weighted, coronal image. The tumour is hypointense, with well defined margins slightly hyperintense. The arrowheads indicate, by comparison, the elevated right sylvian fissure. d) MRI, T2-weighted image. The tumour is markedly hyperintense. e, f) Right carotid angiogram. Elevation and displacement at the middle cerebral artery, produced by the temporal component of the tumour. No abnormal vessels are seen.

At MRI, the tumour tissue is usually slightly hypointense in T1 and definitely hyperintense in T2 (Fig. 26); the lesion usually appears homogeneous with well defined margins; enhancement may be present in nodular or diffuse form. Calcification is present in about 20% of astrocytomas and is better recognized with CT.

Differential diagnosis is relatively easy and includes other neoplastic lesions such as metastasis, other gliomas and lymphomas, cerebral abscesses, cerebritis and ischemic lesions.

Angiography, when performed, usually only shows vessel displacement due to mass effect and no tumour vascularization.

Glioblastoma and anaplastic astrocytoma
Glioblastoma is a highly malignant tumour with a mean survival of months and represents about 15-20 % of all intracranial tumours.
The gross pathological appearance of this tumour is that of an infiltrating lesion, with areas of necrosis and haemorrhage, neovascularization and surrounding oedema.

The CT appearance is very variable in relation to the macroscopic anatomy of the tumour; basically the dens it y is inhomogeneous, with hypodense and hyperdense areas mixed togheter, irregular margins, surrounding oedema and marked inhomogenous enhancement.
At MRI (Fig 27) a wide heterogenicity of pictures is encountered, due to the possible presence of haemorrhage, necrosis and solid components. The tumour is usually hypointense in T1 with possible changes associated with the paramagnetic effects of blood and hyperintense in T2 with the same haemorrhagic changes. Enhancement is usually marked and inhomogenous. Angiography, when performed, shows neovascularization with pathologic vessels and early draining veins.

Oligodendroglioma
Oligodendrogliomas represent about 5 % of all glial tumours, they originate from the oligodendroglia, are highly cellular, infiltrating, with relatively well defined margins and high incidence of calcifications (about 75 %) (Fig. 28). At CT and MRI their appearance is not specific and does not differ much from the other glial tumours apart from the much higher

a    b   c    d  e   Figure 27. Glioblastoma multiform a) MRI, T1-weighted image. Large frontal tumour, inhomogeneous with hypointense area and scattered hyperintensities due to haemorrhage within the tumour. The arrows indicate the middle cerebral artery. b, c) MRI, axial and coronal T1-weighted image following Gd-injection. Marked inhomogenous irregular enhancement of the most peripheral part of the tumour. d, e) Right carotid angiography, early and late phase. Displacement of vessels due to mass effect, neovascularization and earl y draining veins.

incidence of calcification, which is better demonstrated on CT.

Other tumours
Other rare supratentorial intraaxial tumours are subependymomas, choroid plexus papillomas, colloid cysts, ganglioglioma and gangliocytoma, tumours originating from the pineal germ cell, lymphomas and metastasis.

Extraaxial infratentorial tumours

These are tumours located under the tentorium, outside the brain stem and the cerebellum. Symptoms may be due to dysfunction of the nervous structures of the brain stem and cerebellum and to signs of raised intracrainal pressure due to hydrocephalus.

Neuroradiological diagnosis
Plain radiographs are not now necessary although they may show signs of raised intracranial pressure, rarely a direct sign of calcification within the tumour or focal bony changes in the posterior fossa may be found.

a     b Figure 28. Left frontal olidogendroglioma a) Plain film of the skull, lateral projection. Arrows indicate scattered opacities due to calcification. b) CT: calcification in the left frontal lobe within an isodense space occupying lesion compressing the left frontal horn.

 

a   b c   d Figure 29. lnfratentorial meningioma a, b, c, d) MRI, sagittal T1, axial T2, sagittal and coronal T1-weighted images with Gd. The tumour is isointense, but is well seen after Gd-injection, due to the marked enhancement.

CT, without and with intravenous injection of contrast is able to detect the majority of lesions except small intracanalicular neurinomas, MRI has the superiority of superb spatial resolution with multiplanar sections and better tissue discrimination.

Angiography is rarely needed, mainly in vascular tumours (haemangioblastomas).

Classification:

Meningiomas
The most frequent sites for infratentorial meningiomas are: the petrous bone, clivus, foramen magnum, tentorium; extremely rare is an intraventricular meningioma of the fourth ventricle.

CT and MR appearance of posterior fossa meningiomas does not differ from that of supratentorial lesions (Fig. 29).

Neurinomas
They represent about 8 % of intracranial tumours. The most frequent infratentorial neurinoma is that of the 8th nerve; much more rare are those of the 7th, most frequently found in patients with neurofibromatosis type 2, and those of the 12th, 9th and 10th cranial nerves.
The CT and MRI appearances are not different from those of the supratentorial tumours. They usually enhance markedly but may have intratumoural cystic components (Fig. 30).

For the tumours of the 8th nerve, plain films of the skull may still be useful to detect enlargement and erosion of the acoustic canal. Much more important is the possibility to detect, with high resolution MRI and use of paramagnetic contrast media, small intracanalicular tumours, before any bone change or growth within the cerebellopontine angle is observed.

Epidermoids and dermoids
Epidermoids, or less properly "epidermoid cysts" and dermoids are benign congenital tumours of developmental origin. Some terminiological confusion exists because some authors improperly call these tumours cholesteatomas. They originate from ectopic ectodermic cells; the most frequent sites are the subarachnoid space at the level of the clivus, the CP angle, the perimesencephalic cistern. Rarely epidermoids and dermoids may be intraventricular.

On CT (Fig. 31) they appear usually isodense with CSF, do not enhance and displace, without infiltrating, the surrounding structures.
On MRI (Fig. 31) they have a signal close to that of CSF both in T1 and in T2-weighted images. Dermoids may have more complex signals, including T1 shortening and T2 shortening due to the presence of paramagnetic substances.

a    b c Figure 30. arge right acoustic neurinoma a, b) MRI, T1 and T2-weighted images. he tumour is isointense in T1 and inhomogeneously hyperintense in T2. c) MRl, TI-weighted image following Gd-injection. Marked inhomogeneous enhancement of the lesion.

Lipomas
Lipomas are benign tumours of maldevelopmental origin. They are usually incidental asymptomatic findings. The most common site is the uadrigeminal and perimesencephalic cistern.

At CT they appear hypodense, with the density of fat; sometimes they ontain calcification.

On MRI they are hyperintense in T1 and iso-hypointense in T2.

Chordomas
These tumours originate from remnants of the embryonic notochord. They usually arise from the clivus at the level of the spheno-occopital synchondrosis; more rarely from the petrous bone. They produce

a   b c Figure 31. pidermoid in the left cerebellopontine angle a) CT: hypodense space occupying tructure in the left CP angle, displacing the pons and cerebellum. b, c) MRl, T1 and T2-weighted image. The lesion has a signal very dose to that of CSF.

destruction of bone and on CT appear hypodense with hyperdense inclusions representing fragments of destroyed bone.
On MRI they are hypointense in T1, hyperintense in T2, and usually how marked enhancement following Gd-injection.
Other more rare extraaxial tumours of the posterior fossa are choroid lexus papillomas, glomus jugulare tumours and metastases.

lntraaxial infratentorial tumours

As for extraaxial tumours, intraaxial infratentorial tumours may present with focal signs due to impairment of nervous structures of the posterior fossa, brain stem and cerebellum, or with signs of supratentorial hydrocephalus.

a   b c Figure 32. Crebellar astrocytoma. Left hemisphere a, b, c) MR1, T1, T2 and T1-weighted image with Gd. The tumour is inhomogenous, mainly cystic. The solid nodular component enhances following Gd injection.

As a general rule, posterior fossa intraaxial tumours in children are usually primary tumours (brain stem gliomas, cerebellar medulloblastomas or astrocytomas) while in adults most commonly they are secondary tumours (metastases mainly from lung tumours in male, breast tumours in female ).

Classification:

Astrocytoma
Cerebellar astrocytoma accounts for 6-10% of all cerebral tumours and is the most common infratentorial tumour in children. It is a slow growing infiltrating tumour, with significant cystic components in about 60% of cases, mainly localized in the cerebellar hemispheres in 75% of cases. It is more common in the first decade with a peak around the fourth year.

It usually appears as a well defined round lesion with regular margins, the most common histologic variety is the pilocytic one.
On CT the solid components (Fig. 32) of the tumour are isodense and the cystic parts are hypodense. Marked enhancement of the solid component is usually observed, enhancement may occur at the level of the capsule of the cyst. Calcification is found in about 10-20% of cases. MRI (Fig. 32) clearly shows the nodular component, usually isohypointense in T1 and slightly hyperintense in T2. The cyst may be isointense in T1 due to the high protein content and hyperintense in T2. Marked enhancement following Gd injection is usually observed at the level of the parenchymal component.

Medulloblastoma
Medulloblastoma is a highly malignant